Borehole F-88-33, located near Rainy River, Ontario, intersected Cretaceous nonmarine clastic sediments. This is the first documented occurrence in Ontario of Cretaceous sediments associated with the western interior. Lithologie and heavy-mineral analyses were used to differentiate this unit from the overlying Quaternary sediments. Seventy-five species of fossil angiosperm pollen, gymnosperm pollen, spores, megaspores, and algal cysts were recovered from borehole F-88-33 and used to date the pre-Quaternary sediments as late Albian to early Cenomanian. The occurrence of these nonmarine sediments in northwestern Ontario helps to better define the limits of Cretaceous sedimentation in the western interior.